The invention concerns an apparatus for the electrostatic coating of workpieces by the application or spraying of material particles such as powders, fibrous materials and/or paints.
Various devices are known that are suitable for the electrostatic application of pulverulent and fibrous materials and paints. These electrostatic coating devices can be classified into three main groups according to their physical system of charging.
In the first group belong devices with charging electrodes coupled to a very high voltage, 50 to 150 kV. The radius of curvature of this electrode is chosen to be very small (needle point, knife edge) so that the air particles in the vicinity thereof are ionized by the ionizing effect of the electrode (corona discharge), and the fine particles of the material to be sprayed or applied entrain the ionized air particles and thus themselves acquire a charge. The force field formed between the electrode, connected to one pole of a high tension DC source (the other pole of which is grounded) and the object (which is also grounded) to be coated with paint, pulverulent or fibrous material, causes the flow of the charged particles to be directed towards the grounded object.
A disadvantage of these devices is the requirement of relatively high electrical voltages the production of which is complicated and requires expensive voltage sources. In addition, the metallic electrode projects freely and thus presents an increased risk of sparking and fire. Furthermore, it is disadvantageous that the ion wind formed by the ionization of air is harmful to health and can also charge objects disposed in the vicinity of the apparatus, thereby causing unpleasant electrical shocks on touch, as well as presenting a risk of sparking and fire. The non-uniform distribution of charge of the sprayed or scattered particles is a further drawback.
The devices belonging to the second group have, in addition to the high tension electrode for charging, a grounded auxiliary electrode. While flowing through the apparatus, the material to be sprayed passes in contact with the high tension electrode and takes the charge from the intensive field produced by tha auxiliary electrode. This process is to be regarded as modern and progressive in that no outer electrode is used and the operational voltage is relatively low, 20 to 30 kV. However, the scope of utilization is rather limited in that the efficiency is relatively low for materials that are difficult to charge, and have a relatively high specific resistance, principally pulverulent materials.
The devices belonging to the third group have no high tension electrode. The material to be sprayed or applied obtains its charge by frictional electricity; accordingly a high electric potential produced by a particular source of voltage is not required. However, it is a disadvantage that this process is only effective for pulverulent substances, and because of the absence of a high voltage electrode and the force field produced thereby, efficiency is in general lower than the efficiency of the electrode systems.
From German published application No. 1,953,989 a spraying apparatus is known wherein the paint particles are conveyed at least partly in opposition to the ion wind which is produced externally by a pointed charging electrode in front of the mouth of an outlet diffuser, and wherein the charging electrode is arranged at the height of the longitudinal axis of the paint stream and is directed towards the mouth of the spray head. A blunt counterelectrode is constituted by a nozzle ring surrounding the outlet opening. Both electrode arrangements are surrounded by an insulated tube. Since the corona discharge directed to the outlet opening forms a high concentration of atmospheric ions which intersect the stream of atomized paint particles in a direction opposed to that of the movement of the particles, a high charge is transmitted to the paint particles. This known apparatus is, however, provided mainly for liquid paints. Also, in this apparatus the efficiency for materials with higher specific resistance, such as in particular pulverulent materials is still low.